(a) Field of the Invention
The present invention relates to an improvement to the known process of analysis of a specimen of gas by means of a high frequency inductively coupled plasma, to form a source of ions sampled in a mass spectrometer and also concerns an improvement in the apparatus required for carrying out this process.
(b) Description of Prior Art
It is known that the elementary analysis of certain specimens, in particular gaseous components or those which are in solution, is more and more often required to be carried out with very precise means, in order to control the purity of these components in an extremely detailed manner. More particularly, but not exclusively, it is known that some electronic applications such as for the manufacture of highly integrated semi-conductor products, it is indispensable to produce deposits of silicon which are extremely pure and whose composition should remain strictly constant in time. Generally, such deposits are obtained from a silicon compound, such as silane, of formula SiH.sub.4, which is evaporated and deposited under vacuum on a substrate. Now, the quality of the deposit obtained may be strongly modified by the presence in silane, of chemical elements such as lithium Li, sodium Na, iron Fe, calcium Ca, arsenic As, boron B, etc. . ., which modify the semi-conductive properties of the silicon deposit, sometimes substantially, even with relatively low concentrations. It will therefore be realised that it will be of interest, to be able, at any moment, to make a very precise elementary analysis of the gas or of solution used.
For this purpose, in order to achieve this result, it is well known to use a mass spectrometer which is associated with a high frequency inductively coupled plasma, this system being known in the art under the designation ICP/MS, which are the initials of the English expression "Inductively Coupled Plasma Mass Spectrometer". In particular, starting from a torch formed of a tube of quartz which receives a specimen of the gas or the solution, previously converted into a mist, to be analyzed and a plasma producing medium delivered in crown shape around the central injection portion of the specimen in the axis of the torch, the process consists in producing the plasma by means of a high frequency induction device which is coaxial to the torch, thereby producing an excitation of the gaseous mixture at the outlet of the torch and enabling to collect a flow of ions, which is then sent to a sampling device, the latter being associated with a mass spectrometer. In known manner, this device comprises two consecutive conical members which are generally made of nickel, these conical members being disposed one after the other along the axis of the torch, each being provided with an axial orifice to receive a fraction of the flow of ions to be analyzed. The first conical member, which is more opened, is called specimen conical member and is unitary with a support member which is generally cooled by a continuous circulation of an appropriate refrigerating medium, the second conical member, mounted downstream of the latter, being designated under the term sampling conical member. A major fraction of the gas flow is withdrawn between the two conical members, by means of a pump or the like while the remaining portion, after having passed through the second conical member, is introduced into a chamber which is kept under a high vacuum and whose dimensions are sufficient to enable the free passage of the ions which are thereafter collected by the mass spectrometer where they are detected as a function of the ratio of their mass with respect to their charge. Such an apparatus, well known to the specialist in this field, is for example described in the magazine "Analytical Chemistry, Vol. 58, no. 1, Jan. 1986 pages 97 et seq".
Now experience has shown that with specimens containing high concentrations of ions of the material to be analyzed, such as silane SiH.sub.4, the efficient and regular transfer of the ions up to the mass spectrometer through the two interface conical members, faces certain difficulties in view of the fact that in the plasma, the atoms (for example Si) or the ions (for example Si+), rapidly produce a deposit of silicon on the external surface of the conical members themselves, partially blocking the orifices of their axial ducts and thereby very substantially disturbing the analysis which is carried out through the mass spectrometer.
As a matter of fact, in the plasma which is produced, following the solution breakdown in the case of a liquid specimen, in all the cases of atomization of the molecule and excitation of the atoms, whether the specimen is liquid or gaseous, the ions and the atoms are necessarily in gaseous phase because of the temperature of the plasma (generally higher or equal to 5000.degree. K), which is substantially higher than that of the boiling point of the substance, for example equal to 2628.degree. K in the case of silicon. On the other hand, because of the cooling of the nickel conical members, silicon is partially solidified in contact with the latter, thus producing a deposit which is detrimental.